Telecommunications systems, including telephone exchanges and cellular phone sites, are normally provided with a set of back-up batteries to supply power to the system in the event there is a loss of commercial AC power at that location. The back-up batteries, usually large lead-acid batteries, can provide sufficient power to run the system for several hours.
Lead-acid batteries, which occupy a large amount of space in the telecommunications system, must be maintained at full charge at all times, and therefore they are connected to a charging system that also runs off the commercial AC power lines. Should there be a loss of commercial AC power, the batteries will supply the necessary operating voltage to the system, and when commercial power returns, the charging system will not only provide power to the system, but also replenish the energy taken from the batteries. Since the battery charging system is at all times connected to the batteries, its output current waveform must be essentially ripple free and have extremely low output noise voltage (conducted) as defined in NEMA Standard Nov. 16, 1978, publication No. PV 7, Part 4, PV 7-4.03.
Most major telephone exchanges include an engine powered generator to provide alternating current to operate the battery charging system so that the exchange can operate indefinitely, limited only by the available fuel supply. The generator is connected to the exchange by means of a change over switch which may be either manually operated or automatically operated whenever there is a loss of commercial power. The change over switch is necessary to prevent power from the generator from reentering the commercial power grid.
Many smaller telephone exchanges and cellular telephone sites, however, are not equipped with such engine powered generators. Accordingly, such systems are limited in the amount of time they can operate before the energy stored in the back-up batteries is consumed.
It would be desirable to provide an engine driven charging system connected directly to the batteries, thus eliminating the need for a change over switch, but previously existing direct current power sources were not properly regulated nor did they have output power characteristics of such purity that they could be used in telecommunications systems.